This invention relates to composite materials containing inclusions with anisotropic geometrical shapes in general and, more particularly, to composite materials with tailored anisotropic electrical and thermal conductivities.
The ability to control the direction and magnitude of energy flow in one dimension (wire), two dimension (thin film), and three dimension (bulk) solid state components has been considered critical to device performance since the beginning of the electronic age. Diodes and other electronic valves are principal examples. Another example where directionality of thermal and electrical currents affect performance is in thermoelectric devices. The dimensionless thermoelectric figure of merit is a measure of performance and is given by the following equation:
      ZT    =                            S          2                ⁢        σ        ⁢                                  ⁢        T            K        ,where S, σ, K and T are the Seebeck coefficient, electrical conductivity, thermal conductivity and absolute temperature respectively.
The thermoelectric figure of merit is also related to the strength of interaction between the carriers and vibrational modes of the lattice structure (phonons) and available carrier energy states which, in turn, are a function of the materials used in the thermoelectric component. As such, the thermal conductivity, K, has an electronic component (KE), associated with the electronic carriers and a lattice component (KL) associated with thermal energy flow due to phonons. The thermal conductivity can then be expressed as K=KE+KL, and the figure of merit can be expressed generally as
  ZT  =                              S          2                ⁢        σ        ⁢                                  ⁢        T                              K          E                +                  K          L                      .  
Efforts to improve the performance of thermoelectric materials have generally focused on reducing K and maximizing σ. Unfortunately, the two quantities are closely coupled, and changing one typically results in corresponding changes in the other.
Recent efforts to improve thermoelectric performance without sacrificing electrical conductivity have focused on inserting physical obstacles with nano scaled dimensions in thermoelectric structures to presumably impede phonon propagation. Venkata Subramanian et al. U.S. Pat. No. 7,342,169 teach that superlattice structures with nano scale dimensions block phonon transmission while allowing electron transmission thereby raising ZT and is included herein in its entirety for reference. Harmon et al. U.S. Pat. No. 6,605,772 disclose that quantum dot superlattices (QDSL) of thermoelectric materials exhibit enhanced ZT values at room temperature also by blocking phonon transmission and is included in its entirety for reference. Heremans et al. U.S. Pat. No. 7,365,265 and U.S. Publication No. 2004/0187905 disclose that nano scale inclusions on the order of 100 nm in size presumably block phonon transmission in lead telluride (PbTe) and other thermoelectric materials, thereby significantly improving the Seebeck coefficient.
Other physical obstacles to energy flow in solids have been disclosed that affect K. Song et al., Physical Review Letters, Vol. 80, 3831 (1998), demonstrate that an asymmetric artificial scatterer in a semiconductor microjunction deflects ballistic electrons causing nonlinear transport and current voltage (IV) rectification and is included herein in its entirety for reference.
Asymmetric energy flow in materials is a useful property with a multitude of applications not limited to thermoelectric materials.